Watch components

ABSTRACT

Watch component made of a persistent phosphorescent ceramic composite material which is a sintered dense body comprising two or more phases, a first phase consisting of at least one metal oxide and a second phase consisting of a metal oxide containing at least one activating element in a reduced oxidation state, the watch component having a surface which comprises an area which shows phosphorescent emission and an area which does not show phosphorescent emission or which shows phosphorescent emission with an intensity which is lower than that of the emission of the other area.

TECHNICAL FIELD

The present invention relates to watch components made from a ceramiccomposite material which displays high mechanical strength and iscapable to display a persistent phosphorescence.

STATE OF THE ART

Zirconium dioxide (or zirconia, ZrO₂) is one of the most studied ceramicmaterials. ZrO₂ adopts a monoclinic crystal structure at roomtemperature and changes to tetragonal and cubic structures at highertemperatures. The volume expansion caused by the cubic to tetragonal tomonoclinic transformation induces large stresses, and these stressescause ZrO₂ to crack upon cooling from high temperatures. When thezirconia is blended with other oxides, the tetragonal and/or cubicphases are stabilized. Effective dopants include magnesium oxide (MgO),yttrium oxide (Y₂O₃, yttria), calcium oxide (CaO), and cerium oxide(Ce₂O₃).

Zirconia is often more useful in its phase ‘stabilized’ state. Uponheating, zirconia undergoes disruptive phase changes. By adding forexample small percentages of yttria, these phase changes are minimized,and the resulting material has superior thermal, mechanical, andelectrical properties. In some cases, the tetragonal phase can bemetastable. If sufficient quantities of the metastable tetragonal phaseare present, then an applied stress, magnified by the stressconcentration at a crack tip, can cause the tetragonal phase to convertto monoclinic, with the associated volume expansion. This phasetransformation can then put the crack into compression, retarding itsgrowth, and enhancing the mechanical properties. This mechanism is knownas transformation toughening, and significantly extends the reliabilityand lifetime of products made with stabilized zirconia.

It has been described by Drennan and Hanninck (J. A. Ceram. Soc. 1986;69(7): 541-546) that the addition of SrO effectively neutralizes thedetrimental effects of the SiO₂ contaminant in zirconiapartially-stabilized with magnesia. It seems that this effect isobtained through the formation of a glass phase that comprises Si andSr, which is ejected from the bulk of the ceramic during sintering.Cutler and Virkar (J. A. Ceram. Soc. 1991; 74(1): 179-186) have shownthat the addition of SrO and Al₂O₃ to Ce-doped zirconia leads tomechanical strengthening of the zirconia, through the formation ofstrontium aluminate platelets (SrAl₁₂O₁₉). This makes possible theproduction of tough Ce-zirconia with good hardness and strength.SrAl₁₂O₁₉ is also known to show persistent phosphorescent propertieswhen appropriate rare-earth dopants are included in the material.However, the strontium aluminate phase described by Cutler and Virkar isnot phosphorescent, presumably because the Ce is not incorporated intothe strontium aluminate phase and the oxidation state is the non-activeCe⁴⁺ state.

A composite

ceramic

material for optical conversion applications is described in EP 1 588991 A1, one of the phases being a fluorescent phase. The examples in thedocument are focused on a composite of Al₂O₃ and Ce-doped Y₃Al₅O₁₂. Thematerial is obtained by mixing the basis materials and subsequent

fusion

at 1900-2000° C. under vacuum, without any further heat treatment. Thematerials are described to convert blue light between 430 and 480 nm(such as the light emitted by a blue LED) into “white” light. To thisend, the material transmits part of the emitted blue light, whileanother part is converted into yellow light by the Y₃Al₅O₁₂:Ce phase(broad emission spectrum centred around 530 nm). The resulting colourwhich appears as a white light can be adjusted by varying the thicknessof the material.

Document WO 2006/097876 A1 describes a polycrystalline ceramic materialthat comprises a fluorescent material. Ideally, the ceramic is aluminaand the phosphor is a Ce-doped YAG (such as Y₃Al₅O₁₂:Ce³⁺). The ceramicmaterial is intended to convert part of the blue light emitted by a LEDinto yellow light, in order to obtain white light. The material isobtained by mixing alumina and phosphor powders in a slurry, withsubsequent pressing and HIP-sintering. The material comprises typically80 to 99.99 vol. % alumina and 0.01 to 20 vol. % phosphor.

A further ceramic composite for optical conversion is described in WO2008/096301 A1, where both luminescent and non-luminescent phasescomprise Si and N. The application discloses in particular therealization of BaSi₇N and (Ba,Sr)₂Si_(5-x)Al_(x)N_(8-x)O_(x):Eu(obtained by sintering in reducing atmosphere and subsequent washing inacidic solution), the mixing of both components and HIP heat treatmentat 1550° C. and 80 MPa, optionally followed by a heat treatment under N₂at 1300° C.

In WO 2011/094404 A1 a ceramic for optical conversion is described witha fluorescent phase of YAG:Ce with pores of well-controlled size andshape. The formation of the pores is conducted through heat treatmentwhereby pore-forming additives are removed or burned out. The processconsists in a first step of debinding by heating in air at typically1150° C., followed by a second step of sintering in a wet hydrogenatmosphere at 1700-1825° C. Through this process a material with a highdegree of transparency or translucency is obtained.

OBJECT OF THE INVENTION

There is no disclosure of watch components made from a ceramic material,in particular a zirconia-based material, which comprises a persistentphosphorescent phase, in particular a persistent phosphorescent phasethat still emits significant light intensity hours after having beenexcited. In different technical fields there is an interest in obtaininga material which displays a high mechanical stability and at the sametime a persistent luminescence. For instance a persistent phosphorescenteffect is required in applications for watches or for indicators, or asa luminous paint or pigment for e.g. safety applications. In suchapplications it may be appropriate to arrange the luminescent effectaccording to a specific pattern, for technical reasons or motivated bydesign considerations.

It is an object of the present invention to obviate the disadvantages ofthe prior art. In particular it is an object of the present invention toprovide watch components made from a ceramic composite material whichdisplays mechanical strength and is capable to display persistentphosphorescence in a certain pattern.

DESCRIPTION OF THE INVENTION

First, the persistent phosphorescent ceramic composite material of thewatch component is described.

A “persistent phosphorescent material” in the context of the presentinvention means a solid luminescent material which shows light emissionafter the exciting radiation has ceased, with an afterglow on the orderof a few minutes to several hours. This includes, but is not necessarilylimited to, solid luminescent material which shows long phosphorescenceor long persistence corresponding to an afterglow persistent time over500 minutes. The persistent time refers to the time that it takes forthe afterglow to decrease to a luminance of 0.3 mCd/m², which is thelower limit of light perception of the human eye (see, e.g., the“Phosphor Handbook”, S. Shionoya and W. M. Yen, editors, CRC Press 1999,chapter 12).

The persistent phosphorescent ceramic composite material is a densebody. The dense property of the persistent phosphorescent ceramiccomposite material provides the wanted enhanced phosphorescenceperformance and the favorable mechanical properties. What is understoodto be a “dense” body will be described hereafter. In order to achievethe dense character of the persistent phosphorescent ceramic compositematerial, the preparation comprises a densifying step leading to adensified body. The manner of the densifying step is described hereafterin the context of the method of the present invention.

It is important that the densified body has been sintered during itspreparation, since without the appropriate manner of sintering thewanted effect of the present invention, in particular the effect of thepersistent phosphorescence, is not achieved. The appropriate manner ofsintering or heat treatment will be described hereafter in the contextof the method of the present invention.

The sintered solidified body comprises two or more phases, in particulartwo or more crystalline phases. The first phase, which is usually thephase which is present in the highest amount by weight, is the phasewhich is the basis for the mechanical properties, while the second phaseis responsible for the phosphorescent properties of the ceramiccomposite material and shows the type of composition which is usuallycalled a phosphor. The persistent phosphorescent ceramic compositematerial is a composite material. A “composite material” in the contextof the present invention is a bulk composite which means that the two ormore phases are not separated in different parts of the dense body. Forexample, the second phase does not form a thin layer or a coating on thefirst phase.

The first phase consists of at least one metal oxide. Any metal oxidemay be chosen which displays a high level of mechanical stability.Accordingly the metal oxide may be selected from aluminium oxide,zirconium oxide, magnesium oxide, silicon oxide, titanium oxide, bariumoxide, beryllium oxide, calcium oxide and chromium oxide.

Of the possible useful materials for the first phase zirconia ispreferred. Zirconia is highly stable and displays excellent mechanicalproperties. The material is reliable by itself, but according to apreferred embodiment it is used in a stabilized form. This stabilizationcan be achieved through the presence of a further material in a specificamount. This further present stabilizing material may be selected frommaterials derived from cerium, magnesium and yttrium.

Ce-doped zirconia can be used, although an orange-coloured zirconiawould be obtained after treatment in reducing atmosphere.

Mg-doped zirconia may be used as well, but a compromise would have to bemade between the day colour of the composite, luminescence performancesand mechanical properties.

It has turned out that, in the context of the present invention, thepresence of yttria as a dopant in the zirconia leads to a high degree ofmechanical stability and the yttria-doped zirconia is the preferredmaterial for the first phase. Zirconia stabilized with yttria is forinstance produced by the company Tosoh Corporation and a typical productwhich is particularly useful in the preparation of the ceramic compositematerial of the present invention is 3 mol % yttria stabilizedtetragonal zirconia. From the point of view of the inventors,yttria-doped zirconia offers the best potential in terms of day colour,excellent persistence of the phosphor phase and very good mechanicalproperties.

Instead of zirconia, or of yttriated zirconia according to the preferredembodiment, alumina may be used as well as a further preferred species,but this may be less successful, since non-luminescent phases can beformed during sintering. Furthermore, the heat treatments which have tobe conducted in the preparation of the phosphorescent ceramic compositematerial according to the present invention have to be performed at ahigher temperature than for zirconia.

Zirconia may also be used in a doped form and/or with the addition ofpigments to modify its day colour, provided that the zirconia has beenstabilized.

The second phase of the ceramic composite material consists of a metaloxide which contains at least one activating element in a reducedoxidation state.

As the metal oxide material Ca, Ba, Sr and/or Mg-aluminates may be used,or Ca, Ba, Sr and/or Mg silicates, or Ca, and/or Sr aluminosilicates.The preferred metal oxide material in the context of the presentinvention is strontium aluminate. As such strontium aluminate SrAl₂O₄,SrAl₄O₇, SrAl₁₂O₁₉ or Sr₄Al₁₄O₂₅ may for example be used, whereby themost preferred type is Sr₄Al₁₄O₂₅. One of the advantages of Sr₄Al₁₄O₂₅is the circumstance that it is insoluble in water, which may be anadvantage in the method for the preparation of the ceramic compositematerial, since it allows to mill the powder in water and to atomize theresulting slurry. It is a further advantage that is stable at the usualsintering temperature of yttriated zirconia as the preferred embodimentof the metal oxide of the first phase.

The metal oxide of the second phase of the ceramic composite material isdoped with at least one activating element. As such an activatingelement any of the rare earth elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd,Tb, Dy, Ho, Er, Tm, Yb and Lu may be chosen. The addition of any ofthese elements to the metal oxide of the second phase of the ceramiccomposite material, in particular to strontium aluminate as thepreferred embodiment of the metal oxide of the second phase of theceramic composite material, may lead to the wanted phosphorescent effectof the ceramic composite material. The preferred types of the activatingelements are Eu and Dy and it has been found that the most preferredembodiment of the second phase is Sr₄Al₁₄O₂₅ doped with Eu and Dy(Eu²⁺/Dy³⁺ doped Sr₄Al₁₄O₂₅).

Other types of second phase material may be selected. However, the useof such other materials may lead to ceramic composite material withlower performance, since non-luminescent phases can be formed duringsintering.

The amount of the at least one activating element in the metal oxide ofthe second phase can be chosen based on the effect which shall beachieved. A typical content of Eu in strontium aluminate as the metaloxide of the second phase is between 0.05 and 4%, preferably between0.15 and 1% by weight (relative to the total weight of the second phasematerial). This low concentration of Eu leads to a white phosphor powderwhich does not display the usual yellowish tint which is usuallyobtained in commercially available phosphors, but retains a highluminescence and phosphorescence intensity, as exemplified in patentapplication EP 2 626 401 A1. This low concentration of Eu is also ofinterest for the final phosphorescent ceramic composite materialaccording to the present invention intensity in the final phosphorescentceramic composite material.

The amount of the at least one metal oxide of the first phase shall be 5to 95%, relative to the total weight of the materials of the first andthe second phase. In that case the amount of the metal oxide containingthe at least one activating element of the second phase shall be 5 to95%, also relative to the total weight of the materials of the first andthe second phase. The preferred amount of the at least one metal oxideof the first phase shall be 40 to 95% by weight, the further preferredamount shall be 50 to 95% by weight and the most preferred amount shallbe 50 to 80% by weight, in all these cases relative to the total weightof the materials of the first and the second phase, and thecorresponding amounts of the metal oxide containing the at least oneactivating element of the second phase shall be 5 to 60% by weight, 5 to50% by weight and 20 to 50% by weight, respectively, relative to thetotal weight of the materials of the first and the second phase.

The ceramic composite material which is finally obtained is a densematerial. As will be described hereafter, the ceramic composite materialis prepared in a manner that a binder material, which is usually presentin the first step of preparing the green body as the precursor of theceramic composite material, is removed, for example by a heating step,and pores are generated in this step of removing the binder material.These generated pores are subsequently eliminated in subsequent heatingsteps, as will be described in connection with the method for thepreparation of the persistent phosphorescent ceramic composite materialaccording to the present invention.

A “dense body” in the context of the present invention means a bodywhich is essentially without remaining pores, in other words a body inwhich the porous character has been minimized. The dense body displays adensity which is at least 90% of the theoretical maximum density. Thetheoretical maximum density is the density without any remaining pores.It can be estimated by taking into account each phase which is presentand the relative concentration and the density of each phase.

The theoretical maximum density can be calculated based on the knowledgeof the density values of the different components in the compositematerial. For example, in case of a composite material comprising 80% byweight of zirconia containing 3% of yttria as the preferred type ofmaterial for the first phase of the sintered solidified body and 20% byweight of Sr₄Al₁₄O₂₅ as the preferred embodiment of the second phase ofthe sintered solidified body, the theoretical maximum density can becalculated as follows. Based on the density for the yttriated zirconiaof 6.1 g/cm³ and the density for Sr₄Al₁₄O₂₅ of 3.65 g/cm³, a theoreticaldensity for this composition of 5.38 g/cm³ is calculated. With 30% byweight of Sr₄Al₁₄O₂₅ and 70% by weight of yttriated zirconia thecalculated density value is 5.08 g/cm³, and with 50% by weight ofSr₄Al₁₄O₂₅ and 50% by weight of yttriated zirconia the calculateddensity value is 4.57 g/cm³. Such calculated values are an appropriatebasis for the calculation of the density although it must be recognizedthat there is an aspect of uncertainty due to the circumstance that themeasurement of the phase concentration is not precise and the differentphases are not necessarily phase-pure.

It is a preferred embodiment for the dense body that its density is atleast 97% of the theoretical maximum density and a more preferredembodiment that the density is at least 98% of the theoretical maximumdensity.

Since, as mentioned above, there will be a step of removal of the bindermaterial, for example through heating, the binder material itself willnot be identified in the final sintered ceramic composite material.

The watch component of the present invention has a surface whichcomprises an area which is phosphorescent besides an area which is notphosphorescent.

The manner of achieving the area with the phosphorescent properties isdescribed hereafter.

Providing a part of the surface with non-phosphorescent character can beachieved by providing a shielding layer on the surface of the watchcomponent. This shielding layer, or opaque layer, or absorbing layer, ornon-transparent layer, shall be non-transparent to light, so that anytransmission through the shielding layer of external radiation having arange of wavelength that makes it capable to excite the phosphor isprevented, or at least greatly reduced, and/or any transmission of lightemitted from material below the shielding layer in this area would beabsorbed or at least greatly reduced as well.

According to a preferred embodiment of the present invention, aneffective shielding layer shall be made from an appropriate metallicmaterial. Such metallic material may be selected, for example, from thegroup consisting of Au, Ag, Ni, Pt, Pd or alloys of these. The choice ofthe metallic material may depend on the particular combination with theceramic composite material and the resulting optical effect.

According to another embodiment of the present invention, an effectiveshielding layer shall be made from oxides, nitrides, and/or carbidessuch as for example CrN, TiN, or ZrN. The choice of the oxide, nitride,and/or carbide materials may depend on the particular combination withthe ceramic composite material and the resulting optical effect.

According to another preferred embodiment of the present invention, theshielding layer can be realized by forming successive or stacked layersof different materials on the surface of the ceramic composite material,for example by depositing as a first layer a tie layer to enhanceadhesion on the composite and as a second layer a different material,such as a metallic material, on top of the tie layer.

For applying the shielding layer, any conventional manner known in thestate of the art may be chosen. For example, deposition of the metallicmaterial with chemical vapor deposition (CVD), physical vapor deposition(PVD) or a galvanic method is possible. For example, deposition of theoxide, nitride, and/or carbide material with atomic layer deposition(ALD) or physical vapor deposition (PVD) is possible.

The thickness of the shielding layer depends on the material which isused in its preparation. A thickness of at least 10 nm is preferable toachieve the effect of the partial or full inhibition and/or partial orfull absorption of the phosphorescence. For example, tests have shownthat a layer of gold of 10 nm thickness decreases the emitted intensityby 90%, whereas a layer of gold of 40 nm thickness decreases the emittedintensity by 99%. A thickness of more than 0.8 mm shall not be requiredand shall be avoided for leading to unnecessary use of metal materialand giving the watch component a bulky effect. A more preferred range ofthe thickness of the shielding layer shall be 20 to 10,000 nm, even morepreferably between 40 and 4,000 nm.

The shielding layer shall not cover the complete surface of the watchcomponent. There is no particular limitation to the relative area of thetotal surface of the watch component which is covered with the shieldinglayer. The area covered with the shielding layer shall be preferablybetween 1 and 99% of the total surface of the watch component. In thisdefinition the total surface shall be the relevant surface of the watchcomponent in the sense of the surface which shall be accessible to theeye of the observer. Within this range, any particular value of thecovered relative area may be chosen.

The specific pattern of the phosphorescent area may be chosen accordingto any particular requirement. Possible ways to obtain specific patternswill be described hereafter in connection with the context of the methodof the invention.

Besides the application of a shielding layer there are other ways toachieve the wanted feature in the watch component of a surface with aphosphorescent area and a non-phosphorescent area, or of a surface witha phosphorescent area and a less phosphorescent area.

Such a further manner of providing a part of the surface with anon-phosphorescent character involves a treatment step to change theproperties of the phosphorescent ceramic composite material withoutrequiring an additional shielding layer or coating. Different methodsfor this are available. It is possible to change the process for thepreparation of the phosphorescent ceramic composite material so that thefinal product contains areas with different production methods.

It is also possible to conduct the preparation of the watch component ina particular manner and to conduct a treatment of this material duringfabrication and/or after completion of the preparation of the ceramiccomposite material to change its properties according to a particularpattern which neutralizes the phosphorescent effect which is displayedin a certain area, as will be described below.

Next the method for preparing the watch component according to thepresent invention is described.

The description of the preparation of the ceramic composite material isprovided first.

In a first step the materials for the ceramic composite material aremixed as powders.

The manner for mixing the materials is not particularly limited and anyconventional mixing procedure may be used.

The process comprises the mixing of the metal oxide and the phosphorpowder, optionally with stabilizers and binders, and the subsequentshaping of a powder compact (hereafter designated as the green body).

The metal oxide which is used is the metal oxide of the first phase ofthe ceramic composite material described above. The phosphor which isused is the metal oxide containing the at least one activating elementof the second phase of the ceramic composite material as describedabove.

The binders which are optionally used in the preparation of the greenbody are not particularly limited and any material which is suitable inaiding the formation of the green body may be used. Regularly the bindermaterial is an organic material and as such an organic material apolymer material such as, for example, polyethylene glycol (PEG),polyvinyl acetate (PVA), polytetrafluoroethylene, ethylene-vinylacetate, polyethylethacrylate, or poly(methacrylate)-co-ethylene glycoldimethacrylate (PMMA) may be chosen.

The presence of such an organic binder makes the formation of the greenbody easier. Methods for preparing the green body include injectionmolding, tape casting, dry pressing, slip casting, gel casting, directcoagulation casting and extrusion.

Hereafter the green body is treated with heat in a number of differentsteps. It is a first optional step to treat the green body with heat inorder to remove the binder material insofar as it has been presentduring the preparation of the green body. This is called the debindingstep. In this debinding step, pores are generated through the removal ofthe binder material in the heat-treated green body.

When conducted by heating, the debinding step is usually conducted at atemperature of at least 450° C., preferably in an oxidizing atmosphere,the selected temperature and the selected atmosphere depending on thecharacter of the binder material.

Alternatively and depending upon the nature of the binder material, thedebinding step can be carried out by other means, such as for examplecatalytic debinding, or solvent-based debinding.

The debinding step is followed by a first sintering step. The firstsintering step is conducted at a temperature in the range of typically800 to 1600° C. It is the intention of the first sintering step todensify the material. This is achieved by an effect of the heating onthe pores which are present, whereby an elimination of the pores isinitiated. A preferred range for the temperature in the first sinteringstep may be the range between 850 and 1200° C., and a typicaltemperature chosen for first sintering is 900° C. It has been observedthat presintering at high temperatures such as temperatures in the rangeof 1450 to 1500° C. can lead to a slightly decreased performance as aphosphorescent composite material.

The first sintering is preferably conducted in an oxidizing atmosphere.Such an oxidizing atmosphere is regularly an ambient atmosphere, meaningin the presence of air at normal ambient pressure. Instead of air, anenriched oxygen atmosphere may be used as well, but this has notechnical advantage. The debinding and first sintering can be alsoconducted under neutral or reducing conditions, although tests haveshown that this approach is less favourable.

It is possible to combine the two steps of debinding and the firstsintering in one single step. The conditions of this single treatmentstep must comply with the requirements for debinding as well as for thesintering of the first step. This means that the treatment temperaturemust be chosen to be in a range which allows the removal of the organicbinder material under formation of pores of suitable size, or that twotemperatures are chosen for the debinding step and the first sinteringstep without cooling the material to room temperature in between thesteps.

If the wanted effect is not achieved in the one single step, then thedebinding and the sintering under oxidizing conditions are conducted intwo separate steps, but a treatment in one single step is in generalmore favourable.

The first sintering step is followed by a second sintering step which isconducted under reducing conditions. The reducing conditions in thesense of the present invention are such conditions in which a reductionof the activating element or elements in the phosphor as the secondphase of the ceramic composite material is achieved, but at the sametime a reduction of the metal oxide in the first phase of the ceramiccomposite material is avoided.

A usual temperature condition for performing the second sintering stepis the choice of a temperature in the range of 800 to 1600° C., with apreferred temperature of 1350 to 1550° C. and with a more preferredtemperature of 1450 to 1500° C. The duration of the treatment at thistemperature depends on the composition of the composite and thetemperature which has been chosen. A regular treatment condition is aduration of the treatment of 3 hours at a temperature of 1450° C. Thismeans that the effective temperature of 1450° C. is maintained at aconstant level for this duration of 3 hours. The total heat treatmentinvolves a ramp-up period involving a heating of for instance 150° C./hand a cooling period of for instance 4 hours.

The atmosphere for the second sintering step is a reducing atmosphere.Any type of atmosphere which succeeds in reducing the activatingelements in the activated metal oxide phase can be chosen. A typicalreducing atmosphere is an atmosphere of hydrogen in argon (Ar/H₂) or anatmosphere of hydrogen in nitrogen (N₂/H₂).

As an alternative to the treatment in the second sintering step underreducing conditions, a sintering under neutral conditions may bepossible. In that case it is possible to realize a High IsostaticPressing (HIP) treatment in neutral atmosphere (after having sinteredthe material and closed the porosity), or to use spark plasma sintering(SPS) under neutral atmosphere. In both cases, the overall effect is toreduce the material as such treatments are usually performed in graphitemoulds. The colour of the zirconia may be affected under these heattreatments, since it may be reduced during this treatment as well, atleast in a thin surface layer. It should however be possible to removesuch grey or black colour by mechanical polishing or through heattreatment in oxidising atmosphere at T<900° C.

It has turned out that the sintering step under reducing conditions isthe essential step in the preparation of the composite material with thewanted effects of displaying favorable mechanical properties in additionto a wanted degree of phosphorescent properties. Before the secondsintering step under reducing conditions, the material does not displaythe properties of a phosphor and no luminescence is observed. At leastone heat treatment in reducing or neutral atmosphere at a temperature ofat least 800° C. is required to obtain a functional material.

The sintering step under reducing conditions leads to a reduction of theactivating elements in the phosphor. It is an essential feature of theinvention that this step in the preparation only reduces the activatingelements in the second, phosphorescent phase of the ceramic compositematerial. If for instance the metal oxide in the first phase iszirconia, the reduction of the zirconia would lead to a changed color ofthis component. Zirconia has a white color, but the reduced form isgrey-black which would normally be a highly unwanted color change. It isthe surprising effect of the method of the invention that the sinteringstep under reducing conditions preferentially reduces the activatingelements in the phosphorescent phase and not the further componentswhich are present, and in particular not the material of the firstphase.

That zirconia is not reduced is very surprising, since the literatureindicates that a heat treatment of zirconia under reducing atmosphere athigh temperatures results in a blackening of the ceramic. Onepossibility could be that the presence in the ceramic of an activatingelement which is more easily reduced than the ZrO₂, such as the rareearth element(s) (Eu³⁺ for example) contained in the second phase,prevents the colour change of zirconia.

The sintering step under reducing conditions preferably optimizes thedensity of the composite material, whereby the density reaches a maximalvalue and the pores which have been generated during the debinding stepare essentially eliminated and no remaining pores are detected in thefinal product. The maximum density level is obviously achieved under thecircumstance of the complete elimination of the pores.

It is also possible to conduct the different heat treatments, namely thedebinding step, the sintering under oxidizing conditions and thesintering under reducing conditions, in one single heating step. It hasbeen described above that the sintering step under reducing conditionsis the essential step in the preparation of the composite ceramic of thepresent invention. In a combined single heating step, this singleheating step shall in the first place provide the sintering underreducing conditions. Although the possibility to conduct the heattreatments in one single step is technically and economically anattractive possibility, this possibility shall only work when thedifferent functions of the heating steps can be achieved in the singleheating step. In some cases, the function of the debinding step cannotbe achieved under reducing conditions, in particular for certain bindermaterials.

In the previous description of the watch components of the presentinvention, it has been described that these have a surface whichcomprises an area which is phosphorescent, besides or adjacent to anarea which is not phosphorescent or less phosphorescent. In other words,it has been described that the watch components have a surface whichcomprises an area which shows phosphorescent emission, besides oradjacent to an area which does not show phosphorescent emission or whichshows phosphorescent emission with an intensity that is lower than thatof the emission of the other area. In yet other words, it has beendescribed that the watch components have a surface which comprises anarea which shows an emitted phosphorescent intensity, besides oradjacent to an area which does not show an emitted phosphorescentintensity or which shows an emitted phosphorescent intensity that islower than that of the other area. There are different methods forachieving this aspect of the present invention.

For example, the material can be further treated to obtain a desiredesthetical or functional effect, for example by depositing layers onparts of the surface and/or in features by PVD and/or galvanic methods,as for instance described in EP 1 548 524 A1 and EP 1 548 525 A1, or byimpregnation of the green body with metallic salt solutions.

According to a preferred embodiment of the watch components, the surfaceshall comprise an area which is not phosphorescent by being covered witha shielding or opaque or absorbing layer, such as preferably a layermade from one or several metallic materials, and/or from one or severaloxide, nitride, and/or carbide materials. In this preferred embodimentthe shielding layer shall be provided on the surface of the watchcomponent in a specific manner of any shape or pattern according to thedesign of the producer.

There is no particular limitation to the relative area of the totalsurface of the watch component which is not phosphorescent or which isless phosphorescent. This area which is not phosphorescent or which isless phosphorescent shall be preferably between 1 and 99% of the totalsurface of the watch component. In this definition the total surfaceshall be the relevant surface of the watch component in the sense of thesurface which shall be accessible to the eye of the observer. Withinthis range, any particular value of the covered relative area may bechosen.

In the context of the present invention, the patterns are microscopic ormacroscopic, in the sense that they have a typical dimension orcharacteristic size of 10 μm or more, preferably of 100 μm or more, forexample to form digits and/or letters and/or geometric patterns on thesurface of the watch component.

One of the methods which is well known in the state of the art fortreating locally and selectively the surface of a solid body is based onusing the photofabrication or photolithography technique.

Photofabrication is a generic term for techniques in which a photoresistcomprised of a photosensitive or radiation-sensitive resin compositionis coated on the surfaces of process articles such as silicon wafers orother substrates and the coating films formed are patterned byphotolithography.

Photolithography is a process used in microfabrication to transfer ageometric pattern from a photomask to a light-sensitive chemical“photoresist”, or simply “resist”, which is deposited on the substrate.The resist is exposed to light through the photomask, which transformsthe exposed parts. In the case of a so-called positive resist, theportion of the photoresist that is exposed to light becomes soluble to asolvent or surface treatment (the photoresist developer or developmenttreatment), while the portion of the photoresist that is unexposedremains insoluble to the photoresist developer or treatment. In the caseof a negative resist, the portion of the photoresist that is exposed tolight becomes insoluble to the photoresist developer or treatment, andthe unexposed portion of the photoresist is dissolved by the photoresistdeveloper. One obtains thereby a photoresist pattern on the surface,with parts of the surface protected by the photoresist and other partsthat are not covered. These patterns are acting as masks in thefollowing step of layer deposition, or electroforming chiefly usingelectroplating, any of which are applied alone or in combination, todeposit the material or materials forming the shielding layer.

In the photofabrication technique which may be applied in the context ofthe present invention, it is possible to use a positive or a negativephotoresist.

A negative photoresist is usually an organic material which, whenexposed to radiation, undergoes chemical reactions of the type referredto as crosslinking, which reactions result in insolubilizing the exposedphotoresist. The crosslinking reactions are of the type that can beinitiated either by light or by electrons. Because it is possible togenerate electron beams of substantial energy but of small diameter,their use in the generation of small patterns is sometimes preferred tothe use of light. Electron beams also have a much better resolutioncapability than is possible when using an optical mask and lightexposure, and they have a much greater depth of focus. The exposure of aconventional positive photoresist involves solubilization of the exposedareas, and the chemical reactions involved are of the scission ordegradation type, which also require absorption of light or electrons.Because this type of photoresist requires higher flux densities forproper exposure than negative photoresists require, electron beams arenot widely employed to this end.

In both methods of applying the positive resist or the negative resistthe final effect which is achieved is that a surface of the substrate,in the context of the present invention the ceramic composite material,is partly covered with the deposited resist.

In one method which is preferred in the present invention, a shieldinglayer is deposited on the exposed part of the surface, for example byALD, PVD, CVD, or by galvanic techniques. The shielding layer ispreferably made from a metallic material and/or from a oxide, nitride,and/or carbide material and/or from a combination of those. Theshielding layer can be realized by depositing one layer or thin film ofa given material, or by depositing several layers or thin films ofdifferent materials in succession, using the same deposition techniqueor using different deposition techniques. Examples for providing suchlayers are provided in the documents EP1548524 and EP1548525. Thematerial which is used for preparing the layer can be selected from thegroup consisting of Au, Ag, CrN, Ni, Pt, TiN, ZrN, Pd or mixtures ofthese materials. The material layer is deposited on the surface andresist pattern.

After this deposition, the remaining photoresist is removed through thetreatment with a stripping solution. Removing the remaining photoresistleaves a pattern of metal or other material on the surface. With anappropriate material and thin film thickness, light from the environmentis absorbed by the layer and/or the emission from the underlyingcomposite phosphorescent material is absorbed by the layer, resulting inlight emission from part of the surface of the component only.

Further techniques are available as the method according to theinvention which do not involve a photofabrication technique.

According to a further embodiment for the preparation of an area whichis not phosphorescent, a part of the surface of the watch component iscolored, for example by local impregnation with a solution of metallicsalts before sintering and subsequent heat treatment, thereby formingcolouring pigments in the treated zones. With an appropriate pigmenttype, pigment density, and thickness of treated material, light from theenvironment is absorbed and/or the emission from the compositephosphorescent material is absorbed, resulting in light emission frompart of the surface of the component only.

According to yet a further embodiment, a part of the surface is treatedin such a way that the luminescent phase is deactivated. This could berealized by local impregnation with a solution of metallic salts beforesintering and subsequent heat treatment, or by electrochemical methods,or by ionic implantation. With an appropriate treatment, no light isemitted from the treated zones of the composite material, resulting inlight emission from part of the surface of the component only.

It is also possible in another embodiment that part of the surface ofthe watch component is treated in such a way that the luminescent phaseis deactivated and/or that the emitted luminescence is absorbed. Thiscan be realized by local modification of the material by lasertreatment, for example by a laser with femtosecond impulsions. Onepossibility using such a femtosecond laser is to follow and adapt theteaching of WO2013135703A1, in order to ablate the material and/or toform a light-absorbing layer in the treated zones, resulting in lightemission from part of the surface of the component only.

In yet another embodiment, two different materials can be combined torealize the watch component. For example, the watch component can berealized by using a standard ceramic material, and the phosphorescentceramic composite material can be overmolded on the ceramic material toform patterns. The standard ceramic material can be, for example, anyttria-stabilized zirconia, without or with the addition of pigments.

The above methods can be combined at will and/or can be combined withknown means for shaping and/or modifying the surface of ceramicscomponents, and in particular of ceramic watch components, provided thatthey are compatible with each other. As an example, the surface of thecomposite ceramic component can be shaped in order to form features onits visible surface, such as hollows having a given depth and side wallsthat are perpendicular to the visible surface. A first layer can bedeposited on the bottom of the hollows, such as described in EP1548524,and/or a second layer can be deposited on part of the surface outside ofthe hollows, whereby the second layer can be of a different materialthan that of the first layer or identical to it (and, in that case,deposited at the same time as the first layer or not). With appropriatematerials and/or treatments, the emission from the compositephosphorescent material is modified, resulting in light emission frompart of the surface of the component only. As a further example, theentire surface of the component can be colored through impregnation witha metallic salt solution of the green body followed by sintering,thereby preventing light emission. Patterns can then be realized byremoving locally the colored surface region, for example by machiningwith standard techniques, or by ablation with a laser beam, inparticular with femtosecond laser impulsions. As a further example, ashielding layer can be deposited on the entire surface of the component,thereby preventing light emission. Patterns can then be realized byremoving locally the shielding layer, for example by ablation with alaser beam, in particular with femtosecond laser impulsions.

Of course, instead of being completely opaque, the deposited layer canbe deposited in such a way that the emitted light intensity is lower ascompared to the non-covered zones. Alternatively, instead of beingcompletely non-emitting, the treated zones could be treated in such away that the emitted light intensity is lower as compared to thenon-treated zones. In both cases, this results in light emission withdifferent intensities from different parts of the surface of thecomponent. In other words, the watch component has a surface whichcomprises an area which shows phosphorescent emission and an area whichshows phosphorescent emission with an intensity that is different(higher or lower) than that of the emission of the other area. In oneembodiment, the different areas show significantly different emittedintensities, with the smallest difference in emitted intensity betweenthe different areas being at least 20%, preferably at least 50%, morepreferably at least 90%. In another embodiment, the different areas showdifferent emitted intensities that can be slight, with the smallestdifference in emitted intensity between the different areas beingbetween 0.1% and 1%, allowing thus to realize grey levels, in order toform an 8-bit greyscale image for example. In any case, the differencesin emitted intensity should be deliberate and/or controlled, in thesense that they result from an additional feature, layer or treatment ofthe phosphorescent ceramic composite material, as opposed to randomand/or uncontrolled variations in material composition or concentrationof the second, phosphorescent phase in the material.

Through the method of the present invention, a ceramic compositecomponent with excellent mechanical properties and excellent persistentluminescence is obtained, with only part of the surface which emitslight so that there are active/luminescent areas and non-luminescentareas, or with different parts of the surface that emit differentintensities. The obtained material opens many possibilities in terms ofperformances and design, as it is tough, hard, and mechanicallyresistant. It can be used to realize, e.g., exterior parts (watch case,bezel) as well as interior elements (dial, luminescent indexes, hands)of a watch. The presence of a phosphorescent area besides anon-phosphorescent area is a useful manner to provide watches withspecific luminous properties.

FIGURES

FIG. 1. Intensity of emission Lv as a function of time t ofphosphorescent materials used in the present invention comprisingdifferent phases of strontium aluminate with rare earth activatingelements.

FIG. 2. Microstructures of two samples of inventive ceramic compositematerial realized with phosphorescent materials of standard andextrafine granulometry, respectively.

FIG. 3. Influence of initial phosphor grain size on luminescentproperties of phosphorescent materials used in the present invention.

FIG. 4. Microstructures of two samples of phosphorescent compositematerial used in the present invention realized with phosphorescentmaterials without and with a washing step.

FIG. 5. Influence of washing step and treatment temperature onluminescent properties of phosphorescent materials used in the presentinvention.

FIG. 6. Influence of phosphor concentration on luminescent properties ofphosphorescent materials used in the present invention.

FIG. 7. Influence of phosphor concentration on luminescent properties ofphosphorescent materials used in the present invention.

FIG. 8. Comparison of luminescent properties of phosphorescent materialsused in the present invention with a pure phosphor sample.

EXAMPLES

Next the present invention is described in more detail by referring tothe following examples.

Meanwhile the properties of the ceramic composite material weredetermined by the following methods.

The density is measured following Archimedes' method with absoluteethanol. Each sample is measured three times and the mean value iscalculated.

L*a*b* colorimetry measurements are performed after machining andpolishing the sample, on the free side (ie the side that was not incontact with the sample holder during heat treatment), with an apertureof 7 mm on three different locations. The equipment is a MinoltaCM3610d.

The measurements of the toughness were performed by indentation with aKB250 Prüftechnik GmbH equipment. The HV5 indentations were realizedunder a charge of 5 kg applied during 15 s. The toughness was measuredby indentation and evaluated through the formula proposed by K. Niihara(cf Niihara K., A fracture mechanics analysis of indentation inducedPalmqvist crack in ceramics, J. Mater. Sci. Lett., 1983, 2, 221-223):K _(lc)=0.018Hv a ^(0.5)(E/Hv)^(0.4)·(a/c−1)^(−0.5)where E is the elastic (or Young's) modulus (measured value: 220 GPa),Hv is the Vickers hardness in GPa, c is the length of the crack formedfollowing indentation measured from the center of the indentation, and ais the half-length of the diagonal of the indentation.

HV1 microhardness was measured with a LEICA VMHT MOT equipment with acharge of 1 Kg during 15 s. 10 measurements were performed per sample.

The Young's modulus and Poisson ratio were measured by acousticmicroscopy (non-destructive control by ultrasounds). The relativemeasurement uncertainty is 2% for both parameters.

The intensity and decay of the emitted luminescence is measured in ablack chamber on up to six samples with a Pritchard PR-880 photometer.The excitation of the phosphor prior to the measurement is done in thechamber with a standard fluorescent tube. The measurement is performedin three stages: (a) the sample is kept in the black chamber during 8hours prior to charging; (b) the excitation is realized during 20minutes under a D65 fluorocompact lamp at an excitation intensity of 400lux; (c) the emitted luminescence is measured during at least 900minutes with an objective aperture of 3°, one of the samples being areference sample. The sensitivity of the photometer is 0.9 mCd/m², to becompared with 0.3 mCd/m², which is the lower limit of light perceptionof the human eye.

The X-ray diffraction measurements are performed in Bragg-Brentanogeometry with a Cu anode excited with 45 kV electrons. The differentphases are identified on the basis of reference patterns from theliterature, and the phase concentrations (given in wt % in the tablesbelow) are estimated with a typical accuracy of 1 wt %.

Example 1

A sample 1 containing 20 wt % of phosphor has been prepared as follows:

-   -   Mixing 80.0 g of zirconia powder containing 3 mol % of yttria        (TZ-3YS obtained from TOSOH Corporation) and 20.0 g of        Sr₄Al₁₄O₂₅:Eu,Dy powder with 3.0 g of organic binder composed of        1.2 g (40%) PVA and 1.8 g (60%) PEG 20 000 in solution at 50% in        water, with 200 ml distilled water and 1 kg of zirconia balls;    -   Attrition/milling at 400 U/min during 30 min in a zirconia bowl;    -   Filtering of the suspension, rinsing of the balls and bowl with        450 ml IPA, spray-drying of the filtered suspension and rinsing        liquid.

7 g of powder were then pressed in a Ø 40 mm mould. During a first heattreatment, debinding and sintering were performed in one step in afurnace under ambient atmosphere, at 1475° C., with a soak-time of 2 hwith 21 h ramp-up time and 11 h cooling time (total treatment time of 34h).

The obtained pellets were machined and polished. The typical density asmeasured by the Archimedes method was 5.371 g·cm⁻³. Typical colorimetrywas L*(D65)=97.01, a*(D65)=−1.81; b*(D65)=2.21. Phase analysis by X-raydiffraction indicated that the phase ratios of the zirconia (tetragonalto cubic) were not modified with respect to a phosphor-free sample, andthat the phosphor remained in the Sr₄Al₁₄O₂₅ phase. At this stage, thephosphor was not functional and no persistent luminescence was detected.

The second heat treatment was performed in reducing atmosphere, at 1450°C. during 4 h with a ramp-up rate of 150° C.·h⁻¹, under Ar/H₂atmosphere. After this treatment, the samples showed persistentluminescence. The density after treatment was 5.37 g·cm⁻³ and thehardness of the pellet was about 1250 Hv with a toughness of about 5.1MPa·m^(−0.5). The colorimetry was L*(D65)=92.86, a*(D65)=−1.31,b*(D65)=2.53, very close to the colour before sintering

Example 2—Effect of the Strontium Aluminate Phase

The potential of two different strontium aluminates with rare-earth (RE)dopants to obtain a persistent phosphorescent ceramic material that issuitable, e.g., for watch applications was investigated.

Two phases showed suitable performances for such applications: theEu²⁺/Dy³⁺ doped SrAl₂O₄ phase which emits around 520 nm (green) and theless used Eu²⁺/Dy³⁺ doped Sr₄Al₁₄O₂₅ phase which emits around 495 nm(blue). Although the green-emitting phase is most widely used, theblue-emitting material shows very interesting properties in terms ofpersistence and perceived intensity.

Two samples with 20% by weight of active SrAlO material were prepared inthe manner as described in example 1, with a pre-sintering performed at900° C. under air and a sintering in reducing atmosphere at 1450° C. for3 h (sample 2.1 incorporating the green-emission SrAl₂O₄ material andsample 2.2 incorporating the blue-emission Sr₄Al₁₄O₂₅ material). Theresults are given in the following table 1 and in FIG. 1.

TABLE 1 Sr_(x)Al_(y)O_(z) phases ZrO₂ phases Colour Density(Sr₄Al₁₄O₂₅/SrAl₂O₄/ (tetragonal/cubic/ sample Pre-sintering sintering(LAB) (g · cm−3) SrAl₁₂O₁₉) monoclinic) 2.1 900° C. in air 1450° C. in93.9/−5.9/9.2 5.33 0/18/0 60/21/1 Ar/H₂, 3 h 2.2 900° C. in air 1450° C.in 95.7/−3.5/6.3 5.33 18/0/0 60/21/1 Ar/H₂, 3 h

The data prove that the sample with Sr₄Al₁₄O₂₅ showed an emittedintensity that is 10 times higher than for the green emitting material.Although SrAl₂O₄ can be functionally incorporated in a zirconia matrix,it is clearly preferable to use Sr₄Al₁₄O₂₅.

However, the low performances of the SrAl₂O₄ containing samples could bedue to some process steps. For example, as SrAl₂O₄ is water-soluble, itcould be preferable not to use water-based methods for atomisation.

Example 3—Influence of Sr₄Al₁₄O₂₅ Grain Size and Sintering Conditions

The influence of the grain size of the initial phosphor material on theobtained performances was studied for two different sinteringconditions.

The images in FIG. 2 show the microstructures of the samples withstandard granulometry (D_(V10)=1.2 μm; D_(V50)=2.5 μm; D_(V90)=6.4 μm,as in the samples 3.1 and 3.2, at left) and so-called “extrafine”granulometry (D_(V10)=0.1 μm; D_(V50)=1.4 μm, D_(V90)=4.7 μm, as in thesamples 3.3 and 3.4, at right).

The behaviour of the four samples is displayed in table 2 and FIG. 3.

TABLE 2 Sr_(x)Al_(y)O_(z) phases ZrO₂ phases Colour Density(Sr₄Al₁₄O₂₅/SrAl₂O₄/ (tetragonal/cubic/ sample Pre-sintering sintering(LAB) (g · cm⁻³) SrAl₁₂O₁₉) monoclinic) 3.1 1475° C. in air 1450° C. in94.8/−2.5/4.1 5.33 20/0/0 56/23/1 N₂/H₂, 4 h 3.2  900° C. in air 1450°C. in 95.7/−3.5/6.3 5.33 20/0/0 56/23/1 N₂/H₂, 3 h 3.3 1475° C. in air1450° C. in 94.6/−2.2/3.2 5.35 18/0/0 60/21/1 N₂/H₂, 4 h 3.4  900° C. inair 1450° C. in 95/−3/5 5.35 18/0/0 60/21/1 N₂/H₂, 3 h

Although all four samples showed persistent luminescence, it ispreferable in this case to use a strontium aluminate powder withstandard grain size, as the samples with small powder grain size showedsystematically a lower emitted intensity. Furthermore, it appears thatpre-sintering at 900° C. is more favourable than at 1475° C. for thepersistence. Samples with pre-sintering at 1500° C. were comparable tosamples pre-sintered at 1475° C., and samples with sintering in reducingatmosphere at 1500° C. were comparable to samples sintered at 1450° C.(not shown here).

Example 4—Influence of Sr₄Al₁₄O₂₅ Pre-Treatment

The influence of a pre-treatment of the phosphor powder beforeincorporation into the zirconia slurry was studied for differentsintering conditions. This pre-treatment consists in washing the powderin an aqueous acidic solution, such as, for example, a diluted solutionof acetic acid (at a concentration of for instance 10% by mass) at atemperature of 70° C. for a few hours. It is known that the washing stepleads to the removal of an amorphous phase from the powder preparation.

The images in FIG. 4 show the microstructures of the samples withoutwashing (samples 3.1 and 3.2 of example 3, at left) and with anadditional washing step (samples 4.1 and 4.2, at right).

The presentation in FIG. 5 summarizes the behaviour of the two types ofsamples, obtained each under two different conditions. In this figure,the two samples which have not been washed are the samples 3.1 and 3.2described in the example 3.

The properties of two washed samples 4.1 and 4.2 are provided in thefollowing table 3.

TABLE 3 Sr_(x)Al_(y)O_(z) phases ZrO₂ phases Colour Density(Sr₄Al₁₄O₂₅/SrAl₂O₄/ (tetragonal/cubic/ sample Pre-sintering sintering(LAB) (g · cm⁻³) SrAl₁₂O₁₉) monoclinic) 4.1 1475° C. in air 1450° C. in  88/−1.3/1.2 5.38 19/0/0 60/20/1 N₂/H₂, 4 h 4.2  900° C. in air 1450°C. in 94.3/−3.1/4.6 5.38 19/0/0 60/20/1 N₂/H₂, 3 h

Again, all samples show persistent luminescence, but a pre-treatment ofthe phosphor material leads to lower emitted intensities. This effect isnot fully understood and could have several origins (difference in grainsize, for example).

The results also confirm that pre-sintering at 900° C. is morefavourable than an initial treatment at 1475° C. for the persistence.Samples with initial treatment sintered at 1500° C. were comparable tosamples heated at 1475° C., and samples with sintering in reducingatmosphere at 1500° C. were comparable to samples sintered at 1450° C.

Example 5—Effect of the Sr₄Al₁₄O₂₅ Concentration

The influence of the concentration of Sr₄Al₁₄O₂₅ in the compositematerial was studied, with samples comprising 20% by weight, 30% byweight and 50% by weight of phosphor material.

The results of these experiments are displayed in the FIGS. 6 and 7. Inthe FIG. 6, the data for the sample with 20% by weight of phosphormaterial correspond to the data for the sample 3.1 in example 3. In theFIG. 7 the data for the sample with 20% of phosphor material correspondto the data for the sample 3.2 in example 3.

The properties of samples 5.1 and 5.2 with 30% by weight of phosphormaterial and samples 5.3 and 5.4 with 50% by weight of phosphor materialare provided in the following table 4.

TABLE 4 Sr_(x)Al_(y)O_(z) phases ZrO₂ phases Colour Density(Sr₄Al₁₄O₂₅/SrAl₂O₄/ (tetragonal/cubic/ sample Pre-sintering sintering(LAB) (g · cm⁻³) SrAl₁₂O₁₉) monoclinic) 5.1 1475° C. in air 1450° C. in96.3/−3.1/5.4 5.02 30/0/0 49/20/1 N₂/H₂, 4 h 5.2  900° C. in air 1450°C. in 96.2/−3.9/7.4 5.02 Not Not N₂/H₂, 3 h measured measured 5.3 1475°C. in air 1450° C. in 93.7/−3.1/6.3 4.49 49/0/0 28/16/7 N₂/H₂, 4 h 5.4 900° C. in air 1450° C. in 95.5/−5.0/8.8 4.48 50/0/0 33/15/2 N₂/H₂, 3 h

All samples showed persistent luminescence. A higher phosphorconcentration led to a marked increase of the emitted light intensity.Again, pre-sintering at 900° C. is more favourable than an initialtreatment at 1475° C. for the persistence. Samples with initialtreatment sintered at 1500° C. were comparable to samples heated at1475° C., and samples with sintering in reducing atmosphere at 1500° C.were comparable to samples sintered at 1450° C. Sintering times of 3 h,6 h and 9 h also yielded comparable results in terms of emittedluminescence.

The elastic (Young's) modulus decreases with increasing phosphorcontent, from 216 GPa for pure zirconia to 182 GPa for the sample with50 weight % phosphor. The Poisson ratio also tended to decrease withincreasing phosphor content. The toughness was measured at 5.9MPa·m^(−0.5) and 3.9 MPa·m^(−0.5) for 20% and 50% in weight ofSr₄Al₁₄O₂₅, respectively.

Finally, the FIG. 8 displays the emitted luminescence of the 20% and 50%phosphor-zirconia composites treated at 900° C. in air and then at 1450°C. in reducing atmosphere, in comparison with the emitted luminescenceof a pure phosphor sample of the same type as used in example 1 and thefurther samples of the present application (Sr₄Al₁₄O₂₅ film of 160 μmthickness). Remarkably, the intensity is comparable at the outset, andis even higher after 200 minutes and more for the zirconia-phosphorsample than for the pure phosphor. This is an unexpected result andshows the tremendous potential of the approach of the inventors: a toughtechnical ceramic is obtained, with high tenacity and high elasticmodulus, with luminescent properties that are equivalent to those of thepure phosphor powder.

It may be further noted that the measured luminescence is comparable onsamples of 0.6 mm and 2 mm thicknesses.

The invention claimed is:
 1. A watch component made of a persistentphosphorescent ceramic composite material which is a sintered dense bodycomprising at least two phases, wherein the at least two phasescomprise: 50 to 95% by weight of a first phase consisting of at leastone metal oxide and 5 to 50% by weight of a second phase consisting of ametal oxide containing at least one activating element in a reducedoxidation state, the weight proportions of said first and second phasesrelative to the total weight of the two phases, wherein the watchcomponent has a surface which comprises (i) a first area which showsphosphorescent emission and (ii) a second area (a) which does not showphosphorescent emission or (b) which shows phosphorescent emission withan intensity which is lower than that of the emission of the first area.2. The watch component according to claim 1, wherein the metal oxide inthe first phase of the persistent phosphorescent ceramic compositematerial is selected from the group consisting of aluminum oxide,zirconium oxide, magnesium oxide, silicon oxide, titanium oxide, bariumoxide, beryllium oxide, calcium oxide and chromium oxide.
 3. The watchcomponent according to claim 1, wherein the metal oxide in the firstphase of the persistent phosphorescent ceramic composite material iszirconia stabilized with a dopant selected from the group consisting ofCe, Mg and Y.
 4. The watch component according to claim 1, wherein themetal oxide in the first phase of the persistent phosphorescent ceramiccomposite material is zirconia stabilized with yttria.
 5. The watchcomponent according to claim 1, wherein the metal oxide in the secondphase of the persistent phosphorescent ceramic composite material isselected from Ca, Ba, Sr and/or Mg-aluminates, Ca, Ba, Sr and/or Mgsilicates, and Ca, and/or Sr aluminosilicates.
 6. The watch componentaccording to claim 2, wherein the metal oxide in the second phase of thepersistent phosphorescent ceramic composite material is selected fromCa, Ba, Sr and/or Mg-aluminates, Ca, Ba, Sr and/or Mg silicates, and Ca,and/or Sr aluminosilicates.
 7. The watch component according to claim 5,wherein the metal oxide in the second phase of the persistentphosphorescent ceramic composite material is a strontium aluminate. 8.The watch component according to claim 1, wherein the amount of thefirst phase in the persistent phosphorescent ceramic composite materialis 50 to 80% by weight and the amount of the second phase in thepersistent phosphorescent ceramic composite material is 20 to 50% byweight, relative to the total weight of the two phases.
 9. The watchcomponent according to claim 2, wherein the amount of the first phase inthe persistent phosphorescent ceramic composite material is 50 to 80% byweight and the amount of the second phase in the persistentphosphorescent ceramic composite material is 20 to 50% by weight,relative to the total weight of the two phases.
 10. The watch componentaccording to claim 1, wherein the area which is not phosphorescent iscovered with a shielding layer.
 11. The watch component according toclaim 2, wherein the area which is not phosphorescent is covered with ashielding layer.
 12. The watch component according to claim 10, whereinthe shielding layer is made of a metallic material or of an oxide ornitride or carbide material or of a combination of the foregoing. 13.The watch component according to claim 12, wherein the metallic materialis selected from the group consisting of Au, Ag, Ni, Pt, Pd or alloys ofthese materials.
 14. The watch component according to claim 11, whereinthe shielding layer is made of a metallic material or of an oxide ornitride or carbide material or of a combination of the foregoing. 15.The watch component according to claim 14, wherein the metallic materialis selected from the group consisting of Au, Ag, Ni, Pt, Pd or alloys ofthese materials.
 16. A watch component made of a persistentphosphorescent ceramic composite material which is a sintered dense bodycomprising at least two phases, wherein the at least two phasescomprise: 50 to 95% by weight of a first phase comprising at least onemetal oxide selected from the group consisting of aluminum oxide,zirconium oxide, magnesium oxide, silicon oxide, titanium oxide, bariumoxide, beryllium oxide, calcium oxide and chromium oxide and 5 to 50% byweight of a second phase comprising of a metal oxide containing at leastone activating element in a reduced oxidation state, the metal oxidesselected from Ca, Ba, Sr and/or Mg-aluminates, Ca, Ba, Sr and/or Mgsilicates, and Ca, and/or Sr aluminosilicates; the weight proportions ofsaid first and second phases relative to the total weight of the twophases, wherein the watch component has a surface which comprises (i) afirst area which shows phosphorescent emission and (ii) a second area(a) which does not show phosphorescent emission or (b) which showsphosphorescent emission with an intensity which is lower than that ofthe emission of the first area.
 17. The watch component of claim 16,wherein the second area does not show phosphorescent emission, andwherein said second area is covered with a shielding layer, saidshielding layer being made of a metallic material or of an oxide ornitride or carbide material or of a combination of the foregoing,wherein the wherein the metallic material is selected from the groupconsisting of Au, Ag, Ni, Pt, Pd or alloys thereof.
 18. A watchcomponent made of a persistent phosphorescent ceramic composite materialwhich is a sintered dense body comprising at least two phases, whereinthe at least two phases comprise: 50 to 95% by weight of a first phasecomprising a zirconium oxide, and 5 to 50% by weight of a second phasecomprising a strontium aluminate, the weight proportions of said firstand second phases relative to the total weight of the two phases,wherein the watch component has a surface which comprises (i) a firstarea which shows phosphorescent emission and (ii) a second area (a)which does not show phosphorescent emission or (b) which showsphosphorescent emission with an intensity which is lower than that ofthe emission of the first area.
 19. The watch component of claim 18,wherein the strontium aluminate is selected from the group consisting ofSrAl₂O₄, SrAl₄O₇, SrAl₁₂O₁₉ and Sr₄Al₁₄O₂₅.
 20. The watch component ofclaim 19, wherein the second area does not show phosphorescent emission,and wherein said second area is covered with a shielding layer, saidshielding layer being made of a metallic material or of an oxide ornitride or carbide material or of a combination of the foregoing,wherein the wherein the metallic material is selected from the groupconsisting of Au, Ag, Ni, Pt, Pd or alloys thereof.